Conventionally, electronic products use outer housings, sometimes called boxes, to protectively enclose printed circuit ("pc") boards, while typically providing external access to electrical connectors, switches, and/or input/output devices connected to the pc boards.
The sizes of the housings are usually dictated by aesthetics and/or engineering considerations, such as, for example, the need for heat dissipation from the pc board. Traditionally, the need for heat dissipation caused the housings to be larger, i.e., oversized, so as to provide room for air currents to remove unwanted heat from the interior of the housings.
Advances in the engineering of the pc board and components thereon have diminished or eliminated the need for oversized housings, and have permitted the use of housings of compact design. For example, desk-top electronic products such as certain personal computers are now characterized by smaller housings, e.g., "small-footprint" or "slim line" housings.
This reduction in the so-called "package" size of the electronic components is advantageous in a variety of applications. Indeed, as computers and computer systems have increasingly become part of the office environment rather than relegated to back rooms or dedicated "computer offices," computer purchasers and users have embraced the reduction in the size of the housings. A reason is that the smaller housings take up less space on work surfaces, e.g., desk tops, on which the electronic products are frequently placed during use. The more efficient use of space has become a primary consideration in the modern office environment.
With the widespread acceptance of computers in the office environment, it is desirable to have other components of the computer system, e.g., bridges, repeaters, and various other network components, located there as well. Locating network components in the office environment can facilitate network management by the responsible individuals, and can maximize system flexibility though use of distributed facilities.
Unfortunately, such components have heretofore been located within sizable electronic racks or cabinets that are generally regarded, e.g., as too unsightly to be located within the office area. It would be desirable to design such network components so that they can be situated in the office itself, and do so in such a way that they need not occupy valuable office work surfaces.
With the increased use of electronic products in the office environment, electromagnetic interference ("EMI") and radio-frequency interference ("RFI") have become important considerations in the design of such products. Various techniques have been proposed for using housings to shield electrical components on printed circuit boards in order to limit the emission of EMI and RFI created by them, or to protect them from external sources of EMI and RFI that could affect their operation.
Commonly assigned U.S. Pat. No. 4,959,752 discloses an approach to EMI/RFI shielding that involves the use of a substantially continuous, conformal coating on the interior surfaces of the housing. The coating has a first layer of electrically-insulating coating material, and a second layer of electrically conducting coating material over the first layer.
While such a shielding approach is generally suitable for its intended purposes, the use of the coating materials imposes certain constraints on the design of the housings. For instance, since electrical contact between the pc boards and the coating material could cause short circuits on the pc boards, or otherwise impair operation of the electronic products, appropriate techniques must to be employed to avoid such contact and its untoward results.
A simple approach to avoiding such electrical contact is to provide an extra safety margin of space between the pc boards and the housing interiors. Unfortunately, the compact housings described above do not offer the luxury of such additional interior room. Larger housings could be used, but this solution compromises the space efficiencies realized by the compact housings, and would be unacceptable to many purchasers and users.